The present invention relates to a glow plug for a diesel engine which is used for improving the starting characteristics of the diesel engine and, more particularly, to a glow plug for a diesel engine in which a small-diameter portion formed on the distal end side of a sheath type heater (to be referred to as a sheath heater hereinafter) is axially extend to form an elongated portion, thereby coping with an increase in number of valves of the diesel engine.
As a glow plug used for improving the starting characteristics of a diesel engine, ones having various types of structures are conventionally known. In Japanese Patent Laid-Open Nos. 57-182026 and 3-99122, the present applicant has previously proposed a self-temperature control type glow plug having a function as a fast-heating type device and a heat saturation function with which overheating of a heat generating portion is prevented to provide stable heat generating characteristics, by combining resistors made of two types of materials.
More specifically, a glow plug of this type has a sheath heater in which the first helical resistor (to be referred to as the first resistor hereinafter) serving as a heater element and the second helical resistor (to be referred to as the second resistor hereinafter) made of a material having a larger positive temperature coefficient of resistance than that of the first resistor are connected in series and are embedded in a heat-resistant insulating powder in a metal sheath.
In particular, with this sheath heater, if a large power is supplied to the first resistor immediately after the first resistor is turned on, the first resistor quickly generates heat, thus obtaining a function as a fast-heating type sheath heater. Also, in the lapse of a predetermined period of time, the temperature increases to increase the resistance of the second resistor. This makes power to be supplied to the first resistor constant or decrease in order to prevent fusing of the first resistor caused by overheating, thus providing a so-called self-temperature control function. Furthermore, in this structure, since a temperature control means for controlling the power supply need not be provided on a power supply circuit that supplies power to the glow plug, the cost of the entire preheating device can be kept low.
The glow plug of this type is demanded to have so-called overshoot characteristics in which the red heating characteristics at the distal end of the sheath heater are improved so that the glow plug serves as a fast-heating type device and that heat having a temperature lower than the maximum temperature is generated for a predetermined period of time in the after-glow time after the engine is started. For this purpose, there is conventionally proposed a glow plug in, e.g., Japanese Patent Laid-Open Nos. 54-60630, 57-87535, and the like, having a sheath heater as a combination of two types of resistors of different resistances as described above, in which the distal end of the sheath where the first resistor is embedded is formed as a small-diameter portion, so that it has a smaller heat capacity than that of the large-diameter portion of the sheath where the second resistor is embedded.
In the conventional diesel engine glow plug described above, when the sheath heater is to be formed by disposing the first and second resistors in the sheath constituted by the small-diameter portion and the large-diameter portion which are obtained by reducing the diameter of the distal end of the sheath, problems as follows arise.
These problems will be described in detail. The glow plug of this type is mounted in a plug holding hole formed in the cylinder head of a diesel engine, such that the distal end of the sheath heater opposes a combustion chamber or a sub-combustion chamber with a predetermined projecting amount through an insertion hole formed in the distal end of the holding hole. Recently, the hole diameter of the insertion hole tends to be formed as small as possible and the length of the insertion hole tends to increase.
The reason of the above tendency is as follows. In recent years, the diesel engine is required to cope with exhaust gas control, and the numbers of intake valves and exhaust valves must be increased as a part of the countermeasure for this requirement. More specifically, many intake valves and exhaust valves are provided on a cylinder head where the glow plug is mounted. In order to ensure the areas of the valve ports of these valves at the maximum, the mounting portion of the glow plug must be small. Also, the strength of the cylinder head must also be ensured. Accordingly, in this cylinder head, the holding hole for mounting the glow plug, in particular the insertion hole for inserting the distal end of the sheath heater, must be formed as small as possible while it must be formed long.
In view of these demands for the cylinder head, on the glow plug side, the length of the small-diameter portion at the distal end of the sheath must be increased to be larger than that of the conventional small-diameter portion. In the conventional sheath heater structure, however, since the first resistor serving as the heater element is arranged in the small-diameter portion, and the second resistor serving as the controller is arranged in the taper portion or large-diameter portion, if the small-diameter portion is merely extended, desired heat generating characteristics as the sheath heater cannot be obtained.
More specifically, in the structure in which the small-diameter portion of the sheath is extended and the first resistor is arranged on the distal end side of the extended small-diameter portion and the first resistor is connected to the second resistor arranged in the taper portion or large-diameter portion of the sheath at a predetermined space through a linear electrical connecting portion, control of the power supply by means of the second resistor is delayed depending on the size of the space between the first and second resistors, and the temperature generated by the sheath heater can be excessively, undesirably increased.
In the sheath heater proposed in Japanese Patent Laid-Open No. 57-87535, the first resistor is arranged in the small-diameter portion of the sheath, the second resistor is arranged in the large-diameter portion of the sheath, and the first and second resistors are connected to each other near the taper portion that couples the small-diameter and large-diameter portions of the sheath. In this structure, since the elongated small-diameter portion is heated by the first resistor, efficient red heating at the distal end of the sheath cannot be achieved. In addition, since the electrical connecting portion between the first and second resistors is located at the taper portion whose size tends to vary during formation of the small-diameter portion of the sheath, the heat generating characteristics of the sheath heater vary, thus rendering the precision unstable.
When increasing the length of the small-diameter portion of the distal end of the sheath, the first resistor may be simply arranged in this small-diameter portion uniformly at a predetermined pitch. In such a structure, however, heat generated by the first resistor is dispersed to the entire portion of the small-diameter portion, and thus so that the heat generated by the small-diameter portion has a low temperature. In the glow plug of this type, the position where the temperature of heat generated by the sheath heater becomes the maximum ranges between the distal end of the sheath and a position thereof at a length corresponding to almost the diameter of the sheath. In the structure described above, the temperature at the central position of the elongated small-diameter portion becomes the maximum. Then, the temperature distribution at the distal end of the sheath, which is important for the heat generating characteristics of the glow plug, becomes poor, and the red heating of the distal end cannot be achieved, providing a problem in terms of functions.